1. Field of the Invention
This invention relates to integrated circuits and more particularly to digital-to-analog converter integrated circuits.
2. Description of the Related Art
In general, a multi-bit digital-to-analog converter converts a sequence of digital codewords, into one or more analog waveforms. A typical multi-bit digital-to-analog converter updates an analog signal at a uniform sampling interval. The analog signal is generated using multiple individual electrical components (e.g., resistors, capacitors, and/or current sources) that generate individual analog signals for each bit or possible value of an input digital code. The multiple individual electrical components may be coupled to a summing node and the individual analog signals sum to an output analog signal that has a value corresponding to the value of the input digital code. The input digital code may be binary-weighted and the individual electrical components may be sized to generate binary-weighted outputs. Although the binary-weighted digital-to-analog converter is relatively fast, it suffers from poor accuracy.
A thermometer-coded digital-to-analog converter is relatively fast and has relatively high-precision. The thermometer-coded digital-to-analog converter receives a thermometer-coded codeword and includes an individual electrical component for each possible value of the input digital codeword. Referring to FIG. 1, a K-bit (e.g., 8-bit) thermometer-coded digital-to-analog converter includes M=2K (e.g., 256) individual electrical components. In a typical thermometer-coded digital-to-analog converter including M unit elements 106, element mismatches introduce an error in the output analog signal, s(n). The error may include harmonics of the input signal. Referring to FIG. 2, a dynamic-element-matching (DEM) technique reduces the output error of the thermometer-coded digital-to-analog converter. For example, if the value of the input digital code is J, then dynamic element-matching encoder 202, which may be a digital control circuit, controls the data conversion by activating J of the M unit elements. Dynamic element-matching encoder 202 chooses which J unit elements are activated in an order that reduces effects of any element mismatch. Although known dynamic-element-matching techniques may reduce tones in the output analog signal, those techniques do not entirely eliminate the tones and/or may introduce noise into the output analog signal. The noise and tones in the output analog signal may affect the realizable performance of a system using the analog signal and may not be acceptable for some applications. Accordingly, new dynamic-element matching techniques are desired.